1. Field of the Invention
The present invention relates to a high-frequency measuring device that detects high-frequency voltage and high-frequency current and measures that voltage and current by calculating high-frequency parameters such as an impedance, and to a calibration method for such a high-frequency measuring device.
2. Description of the Related Art
In the past, plasma processing systems have been developed that process processed articles such as semiconductor wafers or liquid crystal substrates using a method such as etching by supplying high-frequency electrical power output from a high-frequency power supply device to a plasma processing device. (See Japanese Patent Application Laid-Open Nos. 2007-163308 and 2004-309132, for example.)
FIG. 10 is a block diagram showing the configuration of a typical plasma processing system.
Since an impedance of a plasma processing device 400 fluctuates during plasma processing, there is the risk of a reflected power reflected at an input end of the plasma processing device 400 damaging a high-frequency power supply device 100. Thus, an impedance matching device 200 is typically provided in a plasma processing system A100 between the high-frequency power supply device 100 and the plasma processing device 400, and the impedance matching device 200 carries out a matching operation corresponding to fluctuations in the impedance of the plasma processing device 400. In addition, it is necessary to monitor the impedance of the plasma processing device 400 and high-frequency voltage and high-frequency current and the like at the input end of the plasma processing device 400 during plasma processing.
Monitoring of the plasma processing device 400 is carried out using various high-frequency parameters measured by a high-frequency measuring device 300 provided at the input end of the plasma processing device 400. In addition, the matching operation of the impedance matching device 200 is carried out by control based on various high-frequency parameters measured by a high-frequency measuring device (not shown) provided within the impedance matching device 200. Furthermore, the following explanation is provided using the high-frequency measuring device 300 as an example.
Together with the high-frequency measuring device 300 detecting high-frequency voltage (to be simply referred to as “voltage”) and high-frequency current (to be simply referred to as “current”) and determining a phase difference θ of the voltage and current from the detected values (to be simply referred to as “phase difference”), it also calculates high-frequency parameters such as an effective voltage value V, an effective current value I, an impedance Z=R+jX (equivalent to the impedance of the plasma processing device 400 since the measurement point is in the vicinity of the input end of the plasma processing device 400), a reflection coefficient Γ, a forward power Pf input to the plasma processing device 400, and a reflected power Pr reflected at the input end of the plasma processing device 400 due to impedance mismatch.
The high-frequency measuring device 300 has a capacitor capacitatively coupled to a rod-shaped semiconductor for transmitting electrical power to the plasma processing device 400 and a coil magnetically coupled to the body portion thereof, and detects a voltage v=√{square root over (2)}·V·sin(ωt) with the capacitor or a current i=√{square root over (2)}·I·sin(ωt+θ) with the coil. In addition, the high-frequency measuring device 300 determines the effective voltage value V, the effective current value I and the phase difference θ from a detected voltage v and current i, and then calculates the high-frequency parameters described above using these values according to the following equations (1) to (5). Namely, the high-frequency measuring device 300 is referred to as a so-called RF sensor provided with sensors for detecting the voltage v and current i, and an arithmetic processing circuit for calculating the high-frequency parameters from the detected values of those sensors.
                    R        =                              V            I                    ⁢          cos          ⁢                                          ⁢          θ                                    (        1        )                                X        =                              V            I                    ⁢          sin          ⁢                                          ⁢          θ                                    (        2        )                                          Z          =                      R            +                          j              ⁢                                                          ⁢              X                                      ⁢                                  ⁢                  Γ          =                                                                      (                                                                                    R                        2                                            +                                              X                        2                                            -                      1                                                                                                                (                                                      R                            +                            1                                                    )                                                2                                            +                                              X                        2                                                                              )                                2                            +                                                (                                                            2                      ⁢                      X                                                                                                                (                                                      R                            +                            1                                                    )                                                2                                            +                                              X                        2                                                                              )                                2                                                                        (        3        )                                Pf        =                              VI            ⁢                                                  ⁢            cos            ⁢                                                  ⁢            θ                                1            -                          Γ              2                                                          (        4        )                                Pr        =                  Pf          ⁢                                          ⁢                      Γ            2                                              (        5        )            
In general, since values detected with sensors differ from the correct values due to variations in sensor sensitivity, monitoring devices and measuring devices are typically composed to acquire calibration data that converts detected values to correct values by preliminarily measuring a measured object serving as a reference, and then correcting detected values to correct detection values with the calibration data during actual measurement.
Short-Open-Load-Thru (SOLT) calibration is used to calibrate the voltage v and current i detected by the high-frequency measuring device 300. SOLT calibration consists of first connecting the high-frequency measuring device 300 to a standard having a preliminarily specified true value of an impedance, and then measuring the impedance with the high-frequency measuring device 300. A dummy load having a characteristic impedance of the measurement system (characteristic impedance of a transmission line that transmits high-frequency waves for measurement, and typically an impedance of 50 or 75Ω) and dummy loads having an impedance close to each of an open-circuit impedance (an infinitely large impedance) and a short-circuit impedance (a zero impedance) are used as standards. Next, calibration parameters for calibrating the voltage v and the current i are calculated from an impedance of each standard measured by the high-frequency measuring device 300 and a true value of the impedance of each standard, and then recorded in memory (not shown) of the high-frequency measuring device 300. During actual measurement, each high-frequency parameter is calculated after having corrected the detected voltage v and current i with the calibration parameters recorded in memory.
Since calibration parameters recorded in memory of the high-frequency measuring device 300 are calculated based on impedances approximating limiting values consisting of the open-circuit impedance (the infinitely large impedance) and the short-circuit impedance (the zero impedance) as well as on a characteristic impedance, values can be corrected over an extremely wide range of impedances.
However, since the calibration parameters are determined to that calibration can be carried out over an extremely wide range of impedances, the accuracy of calibration using these calibration parameters is not sufficiently high. Namely, the calibration parameters enable calibration to be carried out at low accuracy over an extremely wide range of impedances. However, in the case of actually using measured values of the high-frequency measuring device 300 to monitor the plasma processing device 400, the accuracy of calibration is required to be high.
For example, since the plasma processing device 400 generates considerable heat when carrying out plasma processing, the ambient temperature of the high-frequency measuring device 300 arranged in close proximity to the plasma processing device 400 also becomes high. If the ambient temperature when carrying out calibration and the ambient temperature during actual measurement differ since the resistance component changes when temperature varies, the accuracy of calibration becomes even lower. In consideration thereof, it is necessary to derive and acquire calibration parameters enabling highly accurate calibration by determining under the same temperature conditions for the measured load and the detection units.
In addition, there are many cases during plasma processing in which the phase difference θ between a detected voltage v and a current i is close to 90°. Thus, the rate of change of cos θ becomes large even if there is only a slight error in the phase difference θ, thereby having a considerable effect on measured values of a resistance component R and a and a forward voltage Pf of an impedance (refer to the previously described equations (1) and (4)). Thus, calibration is required to be carried out with high accuracy in order to suppress error of the phase difference θ.
In addition, measured values of the high-frequency measuring device 300 may also be used for an E chuck controller and the like. An E chuck controller controls the strength of an electrostatic chuck for immobilizing a wafer in the chamber of the plasma processing device 400 based on measured current and voltage values. Thus, it is necessary to reduce error of the measured current and voltage values to an extremely small range, as well as minimize differences in characteristics between devices. Consequently, detected values of the high-frequency measuring device 300 are required to be calibrated at high accuracy.